The `main` element must not appear as a descendant of the `section` element. Correct markup is the first requirement to make user agents and assistive technologies work properly.
Changes the `<section>` element that was wrapping the `<main>` element to a `<div>`.
Props albertomake, ryelle, desrosj, ianbelanger, audrasjb.
Fixes #47066.
git-svn-id: https://develop.svn.wordpress.org/trunk@45942 602fd350-edb4-49c9-b593-d223f7449a82